Gap surface plasmon polaritons enhanced by a plasmonic lens

We numerically investigate the optical field enhancement based on gap surface plasmon polaritons (GSPPs) that are enhanced by propagating surface waves launched by a circular slit at a metal-dielectric interface. The optical field enhancement originates not only from multiple scattering and coupling of GSPPs in the spacer region between two metal layers but also from propagating surface plasmon polaritons (SPPs) launched by a circular plasmonic lens. We find that the combination of the GSPPs and the propagating SPPs launched by the plasmonic lens can achieve extremely strong field confinement, and we find that the surface-enhanced Raman scattering (SERS) enhancement factor can be up to 10(15) at the tip of the equilateral triangular nanostructures. The structure proposed here is expected to find promising applications where strong field enhancement is desired, such as optical sensing with the SERS effect.     

Optimization of loss and gain multilayers for reducing the scattering of a perfect conducting cylinder

Reduction of electromagnetic scattering from a conducting cylinder could be achieved by covering it with optimized multilayers of normal dielectric and plasmonic material. The plasmonic material with intrinsic losses could degrade the cloaking effect. Using a genetic algorithm, we present the optimized design of loss and gain multilayers for reduction of the scattering from a perfect conducting cylinder. This multilayered structure is theoretically and numerically analyzed when the plasmonic material with low loss and high loss respectively is considered. We demonstrate by full-wave simulation that the optimized nonmagnetic gain-loss design can greatly compensate the decreased cloaking effect caused by loss material,which facilitates the realization of practical electromagnetic cloaking, especially in the optical range.     

Subdiffraction‐limited chemical imaging of patterned phthalocyanine films using tip‐enhanced near‐field optical microscopy

A tip‐enhanced near‐field optical microscope, based on a shear‐force atomic force microscope with plasmonic tip coupled to an inverted, confocal optical microscope, has been constructed for nanoscale chemical (Raman) imaging of surfaces. The design and validation of the instrument, along with its application to near‐field Raman mapping of patterned organic thin films (coumarin‐6 and Cu(II) phthalocyanine), are described. Lateral resolution of the instrument is estimated at 50 nm (better than λ/10), which is roughly dictated by the size of the plasmonic tip apex. Additional observations, such as the distance scaling of Raman enhancement and the inelastic scattering background generated by the plasmonic tip, are presented. Copyright © 2016 John Wiley & Sons, Ltd.     

Deep‐UV tip‐enhanced Raman scattering

We report for the first time the tip‐enhancement of resonance Raman scattering using deep ultraviolet (DUV) excitation wavelength. The tip‐enhancement was successfully demonstrated with an aluminum‐coated silicon tip that acts as a plasmonic material in DUV wavelengths. Both the crystal violet and adenine molecules, which were used as test samples, show electronic resonance at the 266‐nm excitation used in the experiments. With results demonstrated here, molecular analysis and imaging with nanoscale spatial resolution in DUV resonance Raman spectroscopy can be realized using the tip‐enhancement effect. Copyright © 2009 John Wiley & Sons, Ltd.     

Nonlinear dynamics of skin potentials in the electrosensory paddlefish

It is known that steady skin potentials are present in fishes due to chloride pumps in the gills and in the skin. We have found previously that these skin potentials can fluctuate and oscillate in the electrosensory paddlefish. Here we show that larger, discharge like potentials can be triggered by applying external electric fields in the water surrounding the fish. These resemble action potentials in nerve cells, but have a longer time scale. Like action potentials, these discharges travel laterally in the skin. They start at the tip of the rostrum and propagate caudally to the tip of the gill covers. They follow the all-or-nothing rule and need some refractory period before they can be evoked again. This is the first time that such discharges, so strikingly similar to action potentials, have been described at the level of a whole organism.     

Multifrequency optical invisibility cloak with layered plasmonic shells

Here, we theoretically suggest the possibility of employing a multilayered plasmonic shell as a cloak for reducing the total scattering cross section of a particle, simultaneously at different frequencies in the optical domain. By exploiting the frequency dispersion of plasmonic materials and their inherent negative polarizability, it is shown, theoretically and with numerical simulations, how covering a dielectric or conducting object of a certain size with this multilayered cloak may reduce its "visibility" by several orders of magnitude simultaneously at multiple frequencies.     

Exploring the origin of tip‐enhanced Raman scattering; preparation of efficient TERS probes with high yield

Tip‐enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, there are several challenges preventing widespread application of this technology, including reproducible fabrication of efficient TERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. It is believed that the coating characteristics at the apex of the tip have a major effect on the near‐field optical enhancement and thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in the efficiency of TERS probes. We argue that the electrostatic field arising from the lightning‐rod effect has a substantial role in the observed TERS effect. This argument is supported by ‘edge‐enhanced Raman scattering’ which is shown for a noble metal film. Furthermore, it is reported that an associated tip‐surface‐enhanced Raman scattering effect can be achieved by using a TERS‐inactive metalized probe on a surface‐enhanced Raman spectroscopy‐inactive roughened surface. This observation can be explained by an interparticle enhancement of the electromagnetic field. Copyright © 2011 John Wiley & Sons, Ltd.     

一种新型介质结构的超传输电磁特性研究

光入射到不同折射率材料的分界面时会很自然地产生反射现象. 在很多的工业应用中, 例如太阳能电池, 衬底的引入会在其表面产生反射损耗. 至今为止, 人们提出很多方法用来克服这一问题, 比较常见的有介质涂层、表面纹理、绝热折射率匹配和散射等离激元纳米粒子. 本文利用二维周期排布的亚波长级硅纳米圆柱阵列来降低衬底表面的反射. 结合辅助微分方程和时域有限差分法对该结构的散射特性进行系统研究, 结果发现, 纳米圆柱粒子能够产生类似于在金属表面发生的超传输现象, 这种现象的发生基于介质衬底耦合Mie共振机理, 该机理能在整个紫外到近红外光谱范围内将能量耦合到衬底中, 从而降低衬底表面的反射; 同时当散射结构被放置在具有高光学态密度的高折射率衬底附近时, 会产生较强的前向散射, 也能有效的减少后向散射即反射的发生. 基于降低衬底表面反射这一目的而言, 我们设计的结构可为实际太阳能电池及光学天线的设计提供参考.     

界面自组装的金/氧化石墨烯复合材料的表面增强拉曼散射行为研究

采用Frens法制备金纳米粒子溶胶,通过界面自组装技术在掺磷的非晶碳衬底表面构筑三维的金/氧化石墨烯/金复合结构.以罗丹明B为探针分子,考察金/氧化石墨烯/金复合材料的表面增强拉曼散射活性.结果表明,由于氧化石墨烯的化学增强和金纳米粒子的电磁场增强的协同作用,在该三维复合材料上获得了很强的罗丹明B拉曼信号.所设计的三维金/氧化石墨烯/金复合材料在生物分析、环境监测、疾病防控、食品安全等领域具有潜在的应用价值.     

Enhanced Raman scattering based on fabry‐perot like resonance in a metal‐cladding waveguide

A liquid‐core metal‐cladding waveguide structure of millimeter scale is designed to enhance Raman signal via the excitation of Fabry–Perot‐like resonance. Theoretically, an oscillating field distribution covering the whole guiding layer is generated by the multireflection at the two metal interfaces. The large detection area covers the whole sample chamber because of the oscillating nature of excited high order modes with concentrated intensity. By adding metal nanoparticles, the Fabry–Perot‐like resonance can be combined with local surface plasmons resonance to further enhance the light‐matter interaction with the target molecules, which is also confirmed by the experimental results. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of tip geometry on the spatial resolution of tip enhanced Raman mapping

In 2013,a breakthrough experiment pushed the Raman mapping of molecules via the tip-enhanced Raman scattering(TERS) technique to a sub-nanometer spatial resolution,going into the single-molecule level.This surprising result was well explained by accounting for the critical role of elastic molecule Rayleigh scattering within a plasmonic nanogap in enhancing both the localization and the intensity level of the Raman scattering signal.In this paper,we theoretically explore the influence of various geometric factors of the TERS system on the spatial resolution of Raman mapping,such as the tip curvature radius,tip conical angle,tip–substrate distance,and tip–molecule vertical distance.This investigation can help to find out the most critical geometric factor influencing the spatial resolution of TERS and march along in the right direction for further improving the performance of the TERS system.     

一种温控的可调表面等离子体光学器件

设计加工了一个三层结构的表面等离子体光学吸收器件,表层结构为规则排列的金质椭圆形微粒。入射电磁波与椭圆形金粒作用激发表面等离子体共振时,由于沿长短轴方向谐振频率不同,使得该器件实现了在近红外谱段两个频率处对入射光近100%的吸收。实验样品用电子束曝光技术加工,实验测量与仿真设计结果一致。此外,由于改变器件表面介质的折射率可以有效调谐器件的谐振特性,通过在器件表面覆盖一向列型液晶层并由温度控制液晶层的折射率,实现了一种温控的可调表面等离子光学吸收器件,调节过程简单可靠并且可重复实现,调节范围达22nm。该器件由于其高吸收效率和可调谐特性,可在太阳能电池以及未来光子集成电路等方面得到重要应用。     

Plasmonic protection of the hot‐electron energy

It is shown, that hot electrons generated in a semiconductor can transfer their excess free energy into an embedded/adjacent plasmonic metallic structure (reservoir), before it is lost irreversibly to phonons in the semiconductor. Since the plasmon–phonon (and plasmon–photon) scattering in the metallic structure could be much slower than the electron–phonon scattering in the semiconductor, free energy of the hot electrons can be this way effectively protected from phonon emission for a significant amount of time. While the cubic point‐dipole crystal is proposed and studied here specifically as the plasmonic reservoir, other plasmonic structures including planar can be employed. It is also shown how the plasmon‐protected energy can by recycled in a novel, 3rd generation solar cell, be employing a planar plasmonic structure that is simultaneously also an electron collector of the cell. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of halloysite nanotubes with deposited silver nanoparticles by methods of optical spectroscopy

Halloysite nanotube composites covered by silver nanoparticles with the average diameters of 5 nm and 9 nm have been studied by methods of optical spectroscopy of reflectance/transmittance and Raman spectroscopy. It has been established that silver significantly increases the light absorption by nanocomposites in the range of 300 to 700 nm with a maximum near 400 nm, especially for the samples with the nanoparticle size of 9 nm, which is explained by plasmonic effects. The optical absorption increases also in the long-wavelength spectral range, which seems to be due to the localized electronic states in an alumosilicate halloysite matrix after deposition of nanoparticles. Raman spectra of nanocomposites reveal intense scattering peaks at the local phonons, whose intensities are maxima for the samples with the silver nanoparticle sizes of 9 nm, which can be caused by plasmonic enhancement of the light scattering efficiency. The results show the ability to use halloysite nanotube nanocomposites in photonics and biomedicine.     

Analysis of damping-induced phase flips of plasmonic nanowire modes

We launch surface plasmons from one end of a silver nanowire by asymmetric illumination with white light and use plasmon-to-light scattering at the nanowire ends to probe spectroscopically the plasmonic Fabry-Perot wire modes. The spectral positions of the maxima and minima in the scattered intensity from both nanowire ends are found to be either in-phase or out-of-phase, depending on the nanowire length and the spectral range. This behavior can be explained by a generalized Fabry-Perot model. The turnover point between the two regimes is sensitive to the surface plasmon round trip losses and thus opens a new possibility for detecting changes of the optical absorption in the nanowire environment.     

Spaser spectroscopy with subwavelength spatial resolution

We propose a method for high-sensitivity subwavelength spectromicroscopy based on the usage of a spaser (plasmonic nanolaser) in the form of a scanning probe microscope tip. The high spatial resolution is defined by plasmon localization at the tip, as is the case for apertureless scanning near-field optical microscopy. In contrast to the latter method, we suggest using radiationless plasmon pumping with quantum dots instead of irradiation with an external laser beam. Due to absorption at the transition frequencies of neighboring nano-objects (molecules or clusters), dips appear in the plasmon generation spectrum. The highest sensitivity is achieved near the generation threshold.     

SERS Platforms of Plasmonic Hydrophobic Surfaces for Analyte Concentration: Hierarchically Assembled Gold Nanorods on Anodized Aluminum

Efficient and homogeneous surface‐enhanced Raman scattering (SERS) substrates are usually prepared using lithographic approaches, physical evaporation, or in situ chemical reduction. However, these approaches are time‐consuming, expensive, and very difficult to upscale. Alternatively, template‐assisted approaches using colloidal suspensions of preformed nanoparticles have become more popular because of their low cost, fast production, and ability to be scaled up easily. One of the limitations of these methods is the dimensions of the structured surfaces. In this context, a new method for designing low‐cost, up‐scalable surface patterns that match building block dimensionality based on anodization of aluminum, enabling a hierarchical organization of anisotropic nanoparticles, is presented. The proposed new technology starts with anodized aluminum oxide with regular parallel linear periodicities. To produce a highly efficient plasmonic surface, gold nanorods are assembled into parallel lines where the long axes of the Au rods are also oriented along the substrate lines, thus inducing reproducible tip‐to‐tip plasmonic coupling with the corresponding generation of highly active hotspots. Additionally, this advanced material presents an inherent hydrophobicity that can be exploited as a method for concentration of analytes on the surface. SERS detection is demonstrated with benzenethiol and 2‐naphtoic acid.     

Insulator—insulator—metal plasmonic waveguide for parasitic scattering suppression in plasmonic optics

We have shown using rigorous electromagnetic simulations that a planar structure consisting of two isotropic dielectric layers can be used to reduce parasitic scattering in plasmonic elements by an order-of-magnitude (to 1–3%). The proposed approach can be used for designing various plasmonic elements such as lenses, Bragg reflectors and plasmonic crystals.     

Optical cloaking of cylindrical objects by using covers made of core-shell nanoparticles

In this Letter, we propose an engineered design of optical cloaks based on the scattering cancellation technique and intended to reduce the observability of cylindrical objects. The cover, consisting of a periodic arrangement of core-shell nanospheres, is designed in such a way to exhibit near-zero values of the real part of the homogenized effective permittivity at optical frequencies. Full-wave numerical simulations, considering the measured data of the dielectric function of the plasmonic material composing the shell, show that the cloak is able to reduce by about 6 dB the scattering cross section of a finite-length cylinder at around 740 THz with a -3 dB fractional bandwidth of about 7%. We show also that this result is not significantly affected by the perturbation of the periodic alignment of the core-shell nanospheres, due to possible fabrication issues or to an amorphous arrangement.